This invention relates to sensors which are used to detect and determine the relative position of a workpiece with respect to the tip of a torch, such as a welding or cutting torch, of the type which is movable in various and optional directions by positioning devices.
Sensors of this general type are provided with an arm which, at one end, carries a sensing body which is intended and adapted to make contact with the workpiece. Generally speaking, a housing is affixed to the end of the torch so that the arm can be swiveled in at least two different planes. The arm is provided with devices which are adapted to cooperate with devices fitted within the housing to generate control signals. These control signals affect the positioning devices in a manner which corresponds to the swiveling displacement of the arm relative to a pre-determined neutral position of the arm.
Sensors of this general type are described in U.S. Pat. No. 3,171,012 of Feb. 23, 1965 in the name of Laural A. Morehead and U.S. Pat. No. 3,997,757 of Dec. 14, 1976 in the name of Shelby Cecil and Dean D. Riggs. Sensors of the type described in these patents are provided with a sensing body which consists of a feeler which picks up the direction of a welding joint. These sensors generate signals which correspond to the position of the welding tip relative to the welding joint. The signals guide the devices which position the tip so that the tip assumes a defined, desired position such as, for example, being always directed toward the center of the joint.
Sensors of this general type can also be used in conjunction with "robots", i.e., devices for the fully automatic handling of tools and workpieces in accordance with a given program. Such robots are presently used extensively in the welding field. Through the use of such a robot, the welding torch is guided from an initial position to the starting point of the welding joint and from there along the welding joint according to a pre-determined program. The sensor detects the actual direction of the welding joint which, as a result of tolerances, generally deviates somewhat from the direction prescribed in the program. The robot is programmed to adapt to the actual conditions.
Such robots are also used extensively in the mass production of workpieces which, in some instances, are conveyed to and placed upon the workplace by another robot. Thus, the workpiece does not always occupy the same position on the workplace and may also be subject to manufacturing and installation tolerances. As a result, the position of the welding joint may be different for each workpiece. Before commencing the welding work, the robot must perform a special localization movement to locate the starting point of the welding joint. To exactly position the workpiece it is generally necessary and required that localization be performed in three directions or planes. To perform this localization, the tip with the sensor is taken to the vicinity of the workpiece and the feeler of the sensor is caused to successively make contact with three selected surfaces of the workpiece in the vicinity of the starting point of the welding joint. Upon making contact with the surfaces, the sensor generates signals which are processed in the robot's computer to determine the position of the starting point. This procedure presupposes that the position of the starting point can be defined with the help of these surfaces; by and large, this is always possible with adequate accuracy.
A feeler which is fitted asymmetrically with respect to the tip, such as the prior art devices shown in the patents referred to above, must be capable of rotating around the axis of the tip so that it can be made effective in the aforesaid three directions of localization. Since one of the degrees of freedom of the robot must be set aside for rotary motion of the tip and therefore cannot be utilized in this operation, the field of application for such robots is limited. In other words, sensors of the type shown in the aforementioned patents have inherent limitations, in that the sensor can only be located on one side of the welding head at any given instant. Thus, such prior art devices are incapable of detecting the proximity of objects which, at any given moment, are located on a side of the welding tip other than the side on which the sensor is presently located.